PYROCRYSTALI.INE ROCKS. 
51 
rals, as pitchstone, obsidian, &c.; or they may be regarded as 
fused feldspars and augite, in which the former may occur in 
obscure crystals.- Their composition may be stated as below: 
Obsidian. 
Pitchstone. 
Silex, 
72 00 
73*00 
Alumina, 
12-50 
14-56 
Potash and soda, 
io-oo 
Oxide of iron and manganese, 
2-00 
1-10 
Lime, 
1-00 
Water, 
0-09 
8-50 
Pearlstone and pumice are products of volcanic action quite 
similar to the foregoing. They are composed of 
Pearlstone. 
Pumice. 
Silex, 
75-25 
72-52 
Alumina, 
12-00 
17-50 
Potash and soda, 
4-50 
3-00 
Oxides of iron and manganese, 1*60 
1-75 
Lime, 
0-50 
Water, 
4-50 
There are other products of igneous action, among which 
siliceous minerals are the common companions, and might be 
regarded as real geological groups. Thus most of the green¬ 
stones, toadstones, and traps furnish varieties of uncleavable 
quartz, as chalcedony, cacholon, cornelian, jasper, siliceous 
sinter, &c. All these varieties are nearly pure silica. Their 
origin is due to the agency of heated water holding silica in 
solution. They are of course of posterior origin to the green¬ 
stones and traps containing them. If we extend this kind of 
grouping a little further, we shall find associated with the 
foregoing a family of minerals which were formerly called 
zeolites, consisting of analcime, laumonite, chabasie, stilbite, 
heulandite, thompsonite, mesotype, phrenite, &c.; calcspar is 
frequently associated with them. They are not confined to the 
greenstones and traps, as talcose and mica slate rarely furnish 
them. But the pyroplastic rocks are the true repositories of 
the zeolites, or according to systematic mineralogy, kouphone 
